These traits are directly related to natural selection—the concept that individuals best adapted for survival will be those that reproduce the most and pass on their traits. In natural selection, some traits may grant individuals an advantage by surviving to reproductive age or producing offspring that also survive. There will thus be more individuals with those traits than with traits that do not confer any advantage, or even confer a disadvantage. Thus, the traits that confer an advantage will become more common than ones that do not.
Life History Traits of Three Species
|Life History Trait||Mouse||Bobcat||Elephant|
|Age at first reproduction||6–8 weeks||1–2 years||10–12 years|
|Average number of offspring per birth||6–8||2–4||1–2|
|Average lifespan||2 years||12 years||60–70 years|
Many life histories consider only the reproductive history of a species: the age at which reproduction may begin, the frequency of reproduction, and the rate at which offspring are born. Among plants, for example, bamboo produces seeds once during its lifespan, then the plant dies. This is also true of many grains. Among animals, butterflies and many spiders mate, lay eggs, then die. Other species of plants and animals reproduce repeatedly and with different levels of frequency. Humans, cows, and giraffes mate and reproduce, yielding usually one offspring at a time. Oak trees, apple trees, and hibiscus plants produce flowers and seeds, continuing to reproduce every year. An oak tree might produce several thousand acorns, but only a handful might sprout to grow into saplings.
Patterns of aging can have significant influences on the other traits. Although members of many species die upon completion of successful mating, others live much longer lives, ensuring their offspring receive the care they require to reach reproductive age themselves. For example, elephant herds often consist of many generations, the oldest ones nurturing and protecting the younger ones, ensuring genetic lines are maintained.
Life and Cohort Tables
A cohort table illustrates a cohort of organisms that are followed through their life stages. It is used to generate a life table, which describes the likelihood (probability) of surviving from one life stage to another.
Life and Mortality Rate Table
|Developmental Stage||Living||Cause of Mortality||Number Deceased||Mortality Rate|
|Larva||100||Loss of food supply||80||80%|
Fecundity and Mortality
Fecundity varies by species. The ability to procreate, and the rate at which a species reproduces, influences the population. Fecundity can be measured in several ways: by the number of births in a population over a certain period of time, by the number of births per female over a given time period, or by births per thousand individuals in a population over a specific time period. Fecundity rates can be high or low. In species that produce frequent litters or clutches, fecundity is high. For example, a female house mouse can begin to mate at 4 to 7 weeks of age and produce a litter every 3 weeks after that. A litter is usually 5 or 6 mice, and that same female can deliver 5 to 10 litters per year. Thus, the fecundity of mice is high. On the other hand, some species reproduce infrequently and have only one offspring at a time, so their fecundity is low. For example, a female elephant does not reach maturity until 10 to 12 years old. A female elephant is pregnant for over eighteen months and generally delivers one or two infants. Elephants reproduce at a rate of one offspring every four years; thus, the fecundity rate of elephants is low. Environmental stresses, such as low food supply, influence fecundity rates for all species.Mortality is considered a natural balance to reproduction. Mortality may be the result of disease, low food supply, extreme weather, predation, age, or other similar factors. The mortality rate of a species may be given as percentages of mortality by age group. For example, the mortality of elephants is 40% for newborns, which means that if a female elephant gives birth to six offspring during her lifetime, only two or three will likely survive beyond infancy. Loggerhead sea turtles, on the other hand, may lay a clutch of more than 100 eggs, but the predation of eggs and hatchlings is high. Of 1,000 eggs, only one offspring may actually survive to grow to reproductive adulthood at 15 to 25 years old.